Several types of generators are known in the art for producing foam from a liquid and a gas, for example a printing paste and air. Most of them consist of a cylindrical stator member having coaxially disposed therein a driven rotor member. The stator inner side and the rotor outer side are provided with rings of radial pins usually having a rectangular cross section. Viewed in axial direction, the stator and rotor rings are alternately disposed. The number of pins in each stator and rotor ring is identical.
In these foam generators, the processing capacity is small in relation to the size of the apparatus. For the preparation of a fine foam of about 200 grams per liter (specific weight approx. 0.2), it has been found in respect of a usual printing paste, that in the known foam generators the ratio (F) of the maximum quantity of paste to be processed, expressed in liters/min., with respect to the generator volume, likewise expressed in liters, has a value of approximately 2.5.
Relative to screen printing techniques, these factors imply that if a printing installation is to process, for example, a maximum of 11 liters/min. of printing paste, each printing unit requires a larger or several smaller types of foam generators to be installed, having a total capacity of 11/2.5=4.4 liters. Regardless of the costs incurred, the relatively large volume of the foam generator has the drawback of resulting in a loss of time before the actual printing procedure can start. This is due to the relatively long passage time of the printing paste, said time increasing to the extent that capacity smaller than the minimum capacity is to be processed. In addition, a considerable loss of printing paste occurs after printing, when the foam generator and the supply and discharge lines thereof are to be cleaned for the next printing operation, said cleaning operation requiring relatively much time.